1. Field of Invention
This invention relates to propagation of orthogonally polarized electromagnetic waves and more particularly to compensating for a loss of orthogonality of polarization due to Faraday rotation in the ionsphere.
2. Description of the Prior Art
A transponder is a device that transmits a signal in response to receiving a signal. A plurality of transponders may, for example, be included in a payload of a communication satellite. A transponder of the satellite typically amplifies and filters a signal received from a first earth station, thereby providing a signal that is transmitted to a second earth station. The satellite's transponder increases the distance over which information may be transmitted from the first earth station.
The signals transmitted to and from the satellite are typically of first and second polarizations, respectively, that are orthogonal to each other. Because of the orthogonal polarizations, the signals may be transmitted to and from the satellite simultaneously, at the same frequency, via one antenna and processed independently of each other. Satellite communication systems that use the orthogonal polarizations of the signals at the same frequency are sometimes known as spectrum reuse systems.
In a spectrum reuse system, an antenna of an exemplary earth station is preferably in a position of alignment with a polarization of an antenna of the satellite's transponder thereby providing a communication link between the exemplary earth station and the satellite. However, the preferable position of the antenna of the exemplary earth station is rotated by ionspheric and atmospheric propagation conditions. Moreover, the rotation of the preferable position is a function of the frequency of a signal and whether the signal is transmitted to the antenna of the transponder or transmitted from the antenna of the transponder. The rotation of the preferable position caused by the change in the propagation conditions is known as Faraday rotation.
In the absence of atmospheric phenomena, such as rain, orthogonally polarized waves that propagate between an earth station and a satellite have an almost entirely predictable Faraday rotation. Therefore, in the absence of rain, elements of an antenna may be rotated in a predetermined manner to compensate for Faraday rotation.
At many small earth stations, only signals within a transmit band of frequencies of a given polarization are transmitted. Additionally, only signals within a receive band of frequencies (different from the transmit band) of a given polarization are received. Since Faraday rotation is a function of frequency, compensation for Faraday rotation at the transmit frequencies is different from compensation at the receive frequencies. There is a need for a simple, economical apparatus for compensating for Faraday rotation at a small earth station of the type described hereinbefore.